Various types of ccntinuous presses are currently known in which two suitably stretched endless filtering webs are used, the material to be drained being introduced between these two webs and progressively pressed by means of a succession of rollers.
When the webs pass over a roller while being sufficiently wound thereon, the pressure necessary for draining the material is generated by the tension of the outer web.
When the webs pass between two juxtaposed rollers, the pressure is generated by squeezing the material between the rollers. Draining in this case is helped by a shearing effect due to the simultaneous deformation or creeping of the material.
All the currently known web presses differ from one another in the way in which the single rollers or the squeezing rollers are positioned, in the way the webs are stretched and guided, in the prior draining systems that they use, etc...
Although they are found to be satisfactory, they nonetheless present certain disadvantages which limit their performances and involve rather high operating costs.
To exert a high pressure and drive them, the filtering webs have to be stretched as much as possible. Consequently, the resistance of the webs and of their junctions limits the possibilities of the press.
Webs are indeed expected to be capable of filtering the material without allowing solids through as well as to withstand very high tensions, two functions which are incompatible.
The web is, as far as possible, chosen to have properties which are a compromise between these two opposite qualities, to the detriment of performances.
When using squeezing rollers between which the webs are squeezed with the material, the choice is even more difficult. It is moreover necessary to use factory-produced endless webs, of which the replacement implies taking the structure of the machine to pieces.
Another disadvantage found with all web presses is that they have no means of holding back the material laterally when the material creeps under pressure before compacting. This implies having to feed the material over a small width in the center of the webs, in order to leave on each side sufficient room for the material to creep without leaking out.
It is also known that, in order to reach rates of dryness close to the pressability limits, it is necessary to use the shearing effect produced by juxtaposed rollers, by going through as many stages of compression as possible, which represents high investments.
Indeed, it means multiplying pairs of juxtaposed strongly built rollers in order to avoid any bending likely to interfere with the squeezing, one roller in each pair having to be necessarily applied against the other by a jack articulation system.